1. Field of the Invention
The present invention relates to drive apparatus with a hydraulic motor having a cubic capacity selector suitable for being moved between a first position corresponding to a large active operating cubic capacity and a second position corresponding to a small active operating cubic capacity, the apparatus comprising control means for controlling the selector, which means comprise a hydraulic control chamber suitable for being fed with fluid under pressure via a control duct for the purpose of urging the cubic capacity selector to move to its second position against the force exerted by first return means, the apparatus further comprising a brake system for braking the hydraulic motor, which brake system comprises brake means mounted to move between a braking position and a brake release position, and a brake release chamber suitable for being fed with fluid under pressure via a brake release duct for the purpose of urging said brake means towards their brake release position against the force exerted by second return means.
The hydraulic motor is, particularly, but not exclusively a motor having radial pistons.
2. Description of the Prior Art
Apparatus of this type is known from Document FR 2 655 090. Conventionally, the brake system constitutes a parking brake which is in the braked position when no fluid pressure is sustained in the brake release chamber. In an operating situation, it is necessary to cause a fluid pressure to prevail in the brake release chamber that is sufficient to release the brake and to feed the motor properly with fluid so as to cause its rotor to turn. By controlling the cubic capacity selector, it is possible to select the cubic capacity of the motor so as to obtain the desired drive torque and the desired speed of revolution.
The large cubic capacity of the motor is the cubic capacity in which, for a given fluid flow rate, the motor can deliver high torque but delivers a low output speed. Conversely, in the small cubic capacity, the output speed is higher, but the torque is lower.
The Applicant has observed that, in certain operating situations, there is risk of the motor racing. This applies, for example, to a motor for moving a vehicle in translation when said vehicle is working on a slope while being held back by the motor. In such a case, if the selector is in its small cubic capacity position, the motor can reach high output speeds because it cannot deliver holding-back torque that is sufficient. For safety reasons, racing must be kept under control, which can be obtained by increasing the holding-back toque of the motor, and thus by switching the selector over to its large cubic capacity position.
An object of the present invention is to limit the effects of the motor starting to race by switching the selector over to large cubic capacity automatically, and therefore unfailingly without action being required from an operator.
The invention achieves this object by the facts that the control duct is suitable for being connected to the brake release chamber for the purpose of putting said brake release chamber into communication with the control chamber, and that the first return means are suitable for returning the cubic capacity selector to its large cubic capacity first position when, while the control chamber is communicating with the brake release chamber via the control duct, the pressure in said brake release chamber becomes lower than a first threshold pressure, whereas the second return means are suitable for returning the brake means to their braking position when the pressure in the brake release chamber becomes lower than a second threshold pressure which is lower than said first threshold pressure.
Thus, it is by the pressure from the fluid in the brake release chamber that it is possible to avoid or to limit the effects of racing. Insofar as the small cubic capacity, for the same fluid flow rate, allows higher speeds than the large cubic capacity and has lower torque that facilitates racing, it is preferable to place the selector in its large cubic capacity position so that with the same fluid flow rate, firstly the motor slows down and secondly (holding-back) torque is increased.
The original idea of the invention thus consists in switching from the small cubic capacity to the large cubic capacity, by using the pressure in the brake release chamber as automatic control means.
The invention differs from the prior art.
In Document FR 2 655 090, controlling brake release is dissociated from controlling the cubic capacity selector.
Document FR 2 673 684 proposes to associate them insofar as the brake release chamber communicates with the control chamber for controlling the cubic capacity selector.
The selector naturally takes up a first position which corresponds to the smaller of the two cubic capacities, and towards which it is continuously urged by the return means. When brake release is performed, the control chamber is automatically fed with fluid under pressure, which tends to move the cubic capacity selector into its large cubic capacity second position.
In order to change the cubic capacity while the motor is operating, i.e. when the brake release chamber is filled with fluid under pressure, it is necessary to urge the selector to move from its small cubic capacity first position against the fluid pressure that prevails in the brake release chamber. For this purpose, it is necessary for the selector to have another control chamber, fed with fluid via another duct. The other duct is itself connected to a feed selector, which is connected either to a fluid feed or else to a fluid return.
In FR 2 673 684, the cubic capacity selector is placed automatically into the small motor cubic capacity position in order to avoid any untimely drive from the drive apparatus when the brake means are in the braking position, without however “overdimensioning” the braking system.
Thus, FR 2 673 684 addresses the problem of starting again after braking, while seeking to prevent it being possible to move the vehicle equipped with the drive apparatus when the brake system is active, i.e. when the brake means are in the braking position.